type virtual_register = int Stdlib.refval _get : int Stdlib.ref -> intval get : int Stdlib.ref -> charval set : int Stdlib.ref -> int -> unitval clear : int Stdlib.ref -> unitval set_nth_bit : int Stdlib.ref -> int -> unitval clear_nth_bit : int Stdlib.ref -> int -> unitval is_nth_bit_set : int Stdlib.ref -> int -> boolval add : int Stdlib.ref -> int -> unit